Small Engine Carburetor Troubleshooting

Small engine carburettors don't handle bad or dirty gas very well, if you own a small engine, you'll encounter carburettor problems at some point. In this guide I'll cover the main problems with small engine carburettors and the solutions.

 

So how do you troubleshooting a small engine carburettor? The most common issue with all small engine carburettors is gumming (old gas), cleaning the carb usually solves the problem.

 

Typical carburettor related problems include:

 

  • Tank - Outlet hole inside the tank blocks with grit stopping or slowing fuel flowing to the carburettor
  • Cap - Cap allows the tank to breath, when the cap vent fails it seals the tank stopping fuel flow
  • Lines - Leak at connection points and on occasion can block stopping fuel flow
  • Tap - Leak, causing fire risk
  • Fuel filter - Block or slow fuel flow to the engine
  • Pump - Fail causing a no start (Not fitted to all mowers)
  • Carburettor - Block, under fuel and over fuel causing no start or poor running
  • Fuel solenoid - Fail, stopping fuel flow (Not fitted to all mowers)
  • Intake manifold - Leak, causing engine surging
  • Refuelling can - Often the source of the dirt
  • Air filter - Can block causing no start or poor running with black smoke

If your not sure you have a fuelling issue - perform a quick Gas Shot Test.

This guide will help you diagnose and fix your fault quickly.

 

Although this guide covers a tractor mower carburettor repair, it will work for all small engine four stroke engines, they all run very similar gas systems.

 

There are many components in the fuel system that can cause issue, but by far the most common fault - Carburettor contamination.

What Is Gumming?

Basically it's old stale gas that turns to a sticky gel, it cloggs up the tiny passages of the finely balanced carburetor. Cleaning usually solves the problem, but if it's bad, don't waste your time cleaning, just go ahead and change out the carburettor.

 

How Does it Happen?

As you know, Ethanol fuel is blended with regular gas, that's not a problem for cars, but it is for small engines. Typically, the small engine is put away for winter with the gas still in the tank.

 

The ethanol blend attracts moisture and the alcohol content in the gas evaporates. The result is gumming and rust - it's a carb killer.

 

To prevent this happening I use a gas stabiliser at the end of this season, mixed with the gas, it'll keep it fresh for up to 2 years, so next spring it's pull and mow.

 

I use a product called Sta-bil gas treatment, 1 ounce treats up to 2.5 gallons, it prevents gumming and cleans the fuel system. It can be used in all gas powered kit including 2 stroke engines. You can use it all season, I only use it at the end of the season and when winterising.

Check out Sta-bil on Amazon.

 

You can read more about Ethanol gas, your tractor mower and how to Stop Gumming here.

 

Of course not all fuelling system faults are gummed up carburettors - running some simple tests will point you in the right direction.

 

mower carburetor

Gumming

Gas turns to a gel and blocks everything up - ya nasty.

When it's bad, I prefer to replace the carb. Cleaning doesn't guarantee that you get it all, then your tearing it down again.

Mower carburetor

Replace

Don't even think about it, Order a new one!

Symptoms Of Carburetor Faults

How my customers describe fuel system faults, one or more of these may sound familiar. Customer complaints include:

 

"Mower stops for no reason"; "I put the mower away for winter and now it won't start"; "Engine runs rough"; "Engine splutters when I cut on a slope"; "Engine dies when I start cutting grass"; "Black smoke from the muffler"; "Engine revving up and down by itself "; "Mower only runs on choke"; "Mower blows white smoke".

 

Ride-on mower engine

Where to start?

Carburetor Fault Finding

At this point, it's assumed that you have ran the Gas Shot Test and Choke System Check, and they both confirmed a fuelling fault. If that is the case your symptom will fit one of the following descriptions:

 

Mower won't start; Runs rough; Blowing black smoke; Starts then dies; Surging; Lacks power; Only runs with choke; Gas leaking into the oil; Blowing white smoke.g up and down by itself "; "Mower only runs on choke"; "Mower blows white smoke".

 

Ride-on mower carburetor

OEM

Carburettors aren't expensive or difficult to fit. Sometimes it's better to just go ahead and replace the whole unit.

Carburettors do wear out and I replace lots of them.

Fuel Solenoid

A fuel solenoid is an electro magnetic valve that simply opens as you turn on and run the mower engine. When the valve is in the open position, it allows gas into the engine.

 

The purpose of the valve is to close at shutdown and prevent gas leaking into the engine which helps prevent engine run-on.

 

Not all mowers will have one fitted, but if you have it will be easy to spot. It lives on the bottom of the carburettor bowl and has an electrical wire and connector fitted.

 

To test the solenoid, turn the ignition on (without starting the motor), locate the sensor and disconnect the wire, now reconnect and listen for the click sound. If you don't hear a click, you could have solenoid failure or a power supply problem.

 

Removing the solenoid is the best way to test, that allows you see it actually open and close. If you have a power supply problem, use a dvom or test light to check for power.

 

If the solenoid fails the mower won't start, and a failing solenoid will cause problems like, only working when it wants to, or shutting down the mower. Changing out the solenoid is easy.

 

Ride-on mower solenoid Ride-on mower solenoid test Ride-on mower solenoid

Test

Remove the connector to test for the click sound, or use a test light to check for power. Briggs and Kohler solenoids shown here.

Fuel Bowl Clean

In some cases you may only need to drain the fuel bowl. In other cases you will need to remove the carburettor and clean it thoroughly. Your carburettor may look different, but the process is the same.

 

In this part of the guide I will drain just the fuel bowl and check fuel flow. You can find your fuel bowl behind the air filter. You don't need to remove the air filter housing to access the bowl.

 

Remember, if your Ethanol gas is older than one month, it's stale, so cleaning the bowl won't make it go. You need to drain the tank, carburettor bowl and fill with fresh gas.

 

I use the Briggs and Stratton oil extractor to remove stale gas and grit from the bottom of the gas tank, it's easier than removing the tank. Check out Briggs extractor on Amazon.

 

If this works out, great!  If not, I wrote this guide, it will walk you through the whole process - Remove & Clean Carburetor.

 

Bowl Cleaning


Ride-on engine

Locate

The carburettor is located behind the air filter and you usually have enough room to work without removing any other components.

Turn the gas off, if you don't have a tap, use a grips to squeeze the line.

Ride-on mower carburetor Ride-on mower carburetor

Remove

This type bowl doesn't have a solenoid.

Remove this bowl by removing the bowl bolt. An o ring gasket is used to seal the bowl to the carburettor.

Usually it stays on the carburettor side and that's OK, you can leave it there.

Clean the bowl, and when refitting use some lube on the o ring seal to prevent pinching.

Ride-on gas solenoid Ride-on carburetor

Remove

This carburettor has a fuel solenoid.

To remove it, disconnect the wire connector and use an open ended wrench between the bowl and solenoid.

Sometimes you can just turn the bowl by hand.

Remember to lube the gasket when refitting the bowl.

Often no matter how careful you are, the bowl gasket will leak gas, if so, the only fix is to replace it.

Ride-on mower carburetor Ride-on mower carburetor

Remove

Remove the fuel bowl drain bolt, which on some models its also the fuel solenoid.

Your bowl may have a bolt or two screws, and in some cases the whole bowl will come off.

Allow the fuel in the carburettor to drain out, catch in a suitable container and have some old rags handy.

If you have any doubts about fuel quality, drain the tank and fill with fresh gas.

Ride-on gas solenoid Ride-on carburetor

Flow

Carburettor bowl type with two screws can be tricky to remove, so if that's your type just remove the solenoid, allow the gas to drain, reassemble and test. Often this is enough to fix the problem.

But before you reassemble check fuel flow on which ever type bowl you have.

Turn the fuel on: If fuel flows - Refit fuel bowl bolt and test mower.

If no fuel flow - Move on to Fuel flow check.

Testing Fuel Flow

If you removed the fuel bowl or drain bolt and found no fuel flowing, or the carburettor needle is leaking gas into the oil. This guide will help you find and fix these problems, it works just the same for walk behind mowers, lawn tractors, ride-on mowers, tillers, snowblowers or any 4 stroke small engine. 

 

Riding lawn mower fuel systems are either gravity feed or pump feed, your mower will be one or the other. You will be able to identify which system you have by following the fuel line from the fuel tank. Go ahead and identify your system, and carry out the checks as directed.

 

Hydro-locking

Gas leaks into the cylinder, when the mowers not in use, filling it right up. This prevents the engine from cranking over, because the piston has no room to move. Some owners think that the battery is flat, and try jump starting without success.

 

Other tell tail signs of hydro-locking are a stink of gas in the garage, gas on the floor of the garage, mysterious loss of gas from the tank and a very high oil level that stinks of gas.

 

Some mowers may start when most of the gas leaks from the cylinder into the oil in the crankcase. The operator then notices lots of white smoke, rough running, stalling and oil leaks.

 

The fix - replace the whole carburettor, because often just replacing the needle seal doesn't work. Fitting a gas tap, and turning off the tap when the mowers not in use will prevent further problems.

 

This guide will show you how to fit a tap and the tools needed - "Fitting a gas tap".

 

Gravity Fuel System

A fuel line from the tank will go to a fuel tap, then a fuel filter and on to the carburettor. (Tap may not be fitted) This system is prone to leaking gas into the oil and causing hydro-locking. 

 

Pump Fuel System

A fuel line from the tank will go to a fuel tap then a fuel filter then a fuel pump and finally to the carburettor (Tap may not be fitted).

 

Flow Test


Ride-on mower gas tank Ride-on mower tank

Remove

A gas tank needs to breath, when fuel leaves the tank it needs to be replaced with air.

A sealed tank will prevent fuel from flowing. Make sure you have gas in the tank.

Remove gas cap and check flow.

Check the fuel tank for grit - the out let hole is small and blocks easily. You may have to remove tank to clean thoroughly. 

Ride-on gas solenoid Ride-on carburetor

Filter

Examine the fuel lines from the tank to the carburettor, checking for kinks or damage.

Some fuel filters will be a see through bottle type, if its dirty - Change it. Arrow to carb.

Ride-on mower carburetor Ride-on mower carburetor

Remove

If you can, remove the bowl - when the float is in the dropped position gas should flow.

Ride-on gas solenoid Ride-on carburetor Ride-on carburetor

Needle

Remove the float and needle, check condition. A worn needle turns pink in colour.

The needle seals the flow of gas when the float is in the up position.

A worn needle can block flow or cause gas to leak into the oil.

When this happens, I prefer to replace the complete carburettor.

Ride-on mower carburetor Ride-on mower carburetor

Spray

Blow some carb cleaner into the needle seat on the carburettor.

Still no flow - Remove & clean carburetor, consider replacing complete unit.

Some carburetors have the seal in the tip of the needle and others have the seal in the carburetor. Carb removed for demo.

Ride-on mower gas filter Ride-on mower starter

Gas Pump

The pumped system is as said very similar. Check that the gas filter supply to the pump is OK.

The fuel pump, operates by the pulsing of crankcase pressure which is supplied by the hose pipe seen in the centre of the lower picture.

Check this pipe is secure and undamaged, sometimes they perish.

To test the pump - Remove the output line on the left and crank over the engine. No fuel flow - Replace pump.

New Carbs

If you've got poor running or carb gumming, it's time to treat her to a shinny new carburettor.

New Carburettor

Remove & Clean Carburettor

Okay, I will assume you have tried cleaning the bowl as per the above guide without success. Now you need to remove the carburettor and clean it.

 

Only basic tools are needed, but a can of carburettor cleaner make life a whole lot easier. In the workshop I use Wd40 cleaner, check it out on Amazon. A container for nuts and bolts, some rags, and take lots of pictures to help you remember where levers, gaskets and springs go.

 

Your carburettor may not be the same as the one used here, but yours will look very similar and the process is the same. Now seat belt on!

 

Ride-on valve lash

Remove 

Remove air filter and engine plastic cover.

mower choke cable

Remove

Remove choke cable.

Ride-on gas line

Remove 

Turn off gas and remove fuel line. If you don't have a gas tap use a grips to gently squeeze the line.

mower intake pipe

Remove

Remove intake pipe.

Ride-on carburetor

Remove 

Unplug solenoid valve and remove both carburettor bolts.

mower choke cable

Photo

Take note of linkage, spring and gasket locations and orientation.

Ride-on carburetor

Remove 

Unplug spring and lever.

mower choke cable

Teardown

Remove the float by sliding the pin out and removing the needle.

When worn the needle seal turns pink. Carburettor kits will include new bowl gaskets and needle seal.

Ride-on carburetor

Remove 

Remove screw.

mower choke cable

Remove

Remove the main jet with a flat screwdriver.

Jets are made from brass which is a soft metal and will damage easily. Be sure the screwdriver is a good fit.

Ride-on carburetor

Remove 

The dirt collects in the jet, it has small port holes which fuel flows through.

mower choke cable

Clean

Clean the jet really well, the port holes may not look dirty, but a build up around them makes them smaller and restricts gas flow.

Use a strand of wire from a wire brush and run it through the holes.

Ride-on carburetor

Check

The bowl gasket may be  distorted or perished. Over-tightening or pinching will cause it to leak.

To avoid damage, lube o ring on reassembly.

mower choke cable mower choke cable

Remove

When removing the fuel/air mix screw, count how many turns it takes to remove, and refit to the same number.

Ride-on carburetor Ride-on carburetor Ride-on carburetor

Spray

Use a good quality carb cleaner and compressed air if available.

Spray all passages and port holes.

mower choke cable

OEM

A new carburettor makes a bit difference, cleaning won't guarantee it runs sweet.

So, if cleaning doesn't work out, go ahead and treat you mower to a new carb.

Ride-on mower oil level too high

Finally

When rebuilding, replace gas filter. Clean your gas can and fill with fresh gas.

If your storing the mower for periods longer than a month, use a gas stabiliser. It will prevent gumming.

Related Questions

What can a dirty carburettor cause? A dirty carburettor can have many symptoms, here's the most common:

 

  • No staring
  • Lack power
  • Hesitation
  • Surging
  • Stalling

Why is my carburettor not getting gas? The most common reason a carburettor isn't getting gas is because of a Dirty carburettor gas bowl, but there are other other possible reasons:

 

  • Gas level too low
  • Gas tap off
  • Blocked gas tank
  • Blocked gas filter
  • Blocked float needle
  • Blocked carburettor jet
  • Blocked gas lines
Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.